Pain is a perceived nociceptive response to local stimuli in the body. The perception of pain at the level of the central nervous system requires the transmission of painful stimuli by peripheral sensory nerve fibers. Upon stimulation of tissue (i.e., thermal, mechanical or chemical), electro-chemical signals are transmitted from the sensory nerve endings to the spinal column, and hence to the brain where pain is perceived.
The cornea is highly innervated with sensory afferents which transmit various painful stimuli to the central nervous system. Pain conditions involving the eye, therefore, can arise in numerous instances, such as: foreign body stimulus, inflammation, dry eye syndrome, accidental trauma, surgical procedures and post-surgical recovery. For example, ocular pain can result from photorefractive keratotomy ("PRK"), a vision correcting, surgical procedure whereby a laser is used to shape the cornea. This process involves the photoablation of Bowman's membrane and the stromal levels of the cornea. As a result, the denuding of the nerve-containing epithelial layers of the cornea can cause some patients to experience pain following laser surgery until the epithelium regenerates.
Various therapies have been attempted for the alleviation of pain. The use of non-steroidal anti-inflammatory drugs (NSAIDs), such as diclofenac, have been developed to treat pain. These agents inhibit cyclooxygenase dependent prostaglandin synthesis. Prostaglandins can modulate pain perception at the level of the central nervous system and systemic administration of NSAIDs is known to provide analgesia. However, the use of NSAIDs can involve undesired side effects including gastrointestinal bleeding and kidney dysfunction.
Local anesthetics are another class of pain modulators which relieve pain by directly inhibiting nerve cellular function. One problem with local anesthetic therapy is that the anesthetics exhibit a short duration of action. Another problem with the use of local anesthetics is that their mechanism of action, non-specific membrane stabilization, can have the undesired coincident effect of also inhibiting biological functions of other cells, such as fibroblasts and surrounding neural cells. Therefore, even though pain sensation can be abated with local anesthetic treatment, healing and normal function of the tissue may be significantly compromised. There is a need, therefore, to discover agents which potently and specifically inhibit the transmission of painful stimuli by sensory afferents, without local anesthetic activity, following topical ocular application.
Other agents have also been suggested for use in treating pain. Such agents include tricyclic antidepressants such as imipramine and desipramine, alpha-2 adrenergic agonists, serotonin uptake blockers, such as prozac, and other analgesics such as paracetamol, as described in U.S. Pat. No. 5,270,050 (Coquelet et al.). Some of these therapies, however, have been associated with side-effects such as dryness of mouth, drowsiness, constipation, and low potencies and efficacies.
Opiates are another class of compounds used to treat pain. Opiates can be administered in a number of ways. For example, opiates can be administered systematically, by intravenous injection or oral dosage, or locally, by subcutaneous, intramuscular or topical application. Systemic administration of opiates, however, has been associated with several problems including dose escalation (tolerance), addiction, respiratory depression and constipation.
"Opioids" is a generic term of art used to describe molecules that produce morphine-like activity in the body. Opioid receptors are membrane proteins which generally cause analgesic responses when bound by opioids. There are three major categories of opioid receptors, designated .mu. (mu), .kappa. (kappa) and .delta. (delta). Other sub-type receptors appear to exist as well. Opioid receptors have been differentiated among each other by the preferential binding affinities of different agonists and antagonists, and by the different responses obtained from each receptor's binding. For example, the full agonist morphine has a ten times greater affinity for the mu receptor than for the delta and kappa receptors. Thus, morphine is a mu agonist (See, Goodman and Gilman's Pharmacological Basis of Therapeutics (8th Edition), Jaffee, Chapter 21: Opioid Analgesics And Antagonists, page 485-492 (1993).) Kappa receptors have also been delineated from the general class of opioid receptors by the fact that mu and delta receptor agonists increase membrane potassium conductance and decrease the duration of presynaptic action potential, whereas kappa receptor agonists decrease voltage-dependent calcium conductance without affecting potassium conductance (Kanemasa, k-opioid agonist U50488 inhibits P-type Ca.sup.2+ channels by two mechanisms, Brain Research, volume 707, pages 207-212 (1995)).
While it is known that opiate analgesics such as morphine relieve pain by activating specific receptors in the brain, recent studies demonstrate the analgesic effects of compounds which act on kappa receptors in peripheral tissue. (See, Joris et al., Opiates suppress carrageenan-induced edema and hypothermia at doses that inhibit hyperalgesia, Pain, volume 43, pages 95-103 (1990); Eisenberg, The peripheral antinociceptiave effect of morphine in a rat model of facial pains, Neuroscience, volume 72, No. 2, pages 519-575 (1996); and Gohschlich, The peripherally acting k-opiate agonist EMD 61753 and analogues: opioid activity versus peripheral selectivity, Drugs Exptl. Clin. Res., volume XX1(5), pages 171-174 (1995)).